Photoelastic processes for examining mechanical stresses

ABSTRACT

A process of visualizing the maximum-shear lines or isostatics by observations under polarized light of the double refractions appearing in a photoelastic material comprises obtaining by means of a noval polariscope device moire images by observing simultaneously the material and a grid for the isostatics for different successive orientations of the plane of polarization. The successive orientations are obtained by simultaneously rotating the plane of the polarized light and the grid. One at least of the material and the grid is observed in the form of an optical image formed in the plane of the other.

United States Patent Paraskevas et al.

on 3,847,481 1 Nov. 12, 1974 PHOTOELASTIC PROCESSES FOR EXAMlNlNG MECHANICAL STRESSES lnventors: Dimitri Paraskevas, Ci'eil; Alain A.

LHermite; Georges F. Bernard, both of Senlis; Jacques E. Guinet, Gouvieux, all of France Assignee: Centre Technique des Industries Mecaniques, Senlis, France Filed: Mar. 29, 1973 Appl. No.: 345,896

Foreign Application Priority Data Apr. 27, 1972 France 72110617 May 24, 1972 France 72.18422 U.S. Cl 356/34, 73/88 A, 356/23, 7

Int. Cl. G011) 11/18 Field of Search 73/88 A; 356/32, 33, 34, 356/35, 169; 350/159 References Cited UNITED STATES PATENTS ORegan 356/34 OTHER PUBLICATIONS Schwallie et al 356/33 Schwallie 356/32 General Radio Experimeter, The Polariscope for Dynamic Stress Analysis, Vol. XXV, No. 1, June 1950.

Primary ExaminerRonald L. Wibert Assistant Examiner-Matthew W. Koren [57] ABSTRACT A process of visualizing the maximum-shear lines or isostatics by observations under polarized light of the double refractions appearing in a photoelastic material comprises obtaining by means of a noval polariscope device moire images by observing simultaneously the material and a grid for the isostatics for different suc:

cessive orientations of the plane of polarization) The successive orientations are obtained by simultaneously rotating the plane of the polarized light and the grid. One at least of the material and the grid is observed in the form of an optical image formed in the plane of the other. 7

18 Claims, 8 Drawing Figures vmminuuv 12 1974 3847.481

SHEET 2 OF 4 ATENTEDHUV 12 I974 sum 3 o 4 SHEET H 0F 4 PATENTED NOV 12 1914 PHOTOELASTIC PROCESSES FOR EXAMINING MECHANICAL STRESSES The present invention relates generally to photoelastic processes for examining mechanical stresses; and more especially relates to a process for visualizing the isostatic lines or lines of maximum-shear force in a body, and also to a device for carrying out such a process.

isostatic lines are envelopes of the directions of the principal stresses in a body. These are the field lines of the principal stresses, and they form two groups of curves, which are at all points orthogonal relative to each other.

Any free contour of a part subjected to plane stresses coincides with an isostatic.

The maximum-shear lines comprises two groups of orthogonal curves, intersecting the isostatics at 45.

It is known that techniques of photoelasticity consist essentially in observing under polarized light the accidental double refractions appearing in a photoelastic material under load. Two groups of conventional techniques can be distinguished, depending on whether one examines by transmission a model of the real structure, made from a suitable transparent material, or whether a film of photoelastic material is applied to the surface of the real structure, and this film is examined by reflection, subject to the same forces as the surface of the structure.

By varying the plane of polarization relative to the material observed, be it a model or a photoelastic film, different networks of isoclinic lines may be visualized, and on the basis of these networks the isostatics or the maximumshear lines may be defined.

Processes and devices are known, enabling direct viv sualization by means of successive superimposed photographs of the isostatics or maximum-shear lines, avoiding the long and painstaking point-by-point tracing, from the networks of isoclinic lines, in the case of examination by transmission of a model of transparent material placed between a polarizer and an analyzer. In this case there is placed against the model being examined, a grid, preferably having a square mesh, whose lines are parallel to the planes of polarization crossed by a polarizer and an analyzer. The polarizer, the grid and the analyzer constitute an assembly which is rotatable relative to the modelsexamined. Successive photographs are then taken with the same exposure time and, after rotation, each time through the same angle, between the polarizer-grid-analyzer assembly, and the model. The superimposed images of the grid thus obtained, and modulated by the successive groups of isoclinic lines, causes watered-silk effects which result in the appearance of two groups of orthogonal curves, which correspond precisely with the maximum-shear lines. The same process applied by arranging the lines of the grid at 45 to the planes of polarization of the polarizer and analyzer, enables direct visualization of the isostatics.

Despite its advantages, practical exploitation of this process has been held back by the difficulties in braking the device and of other operating conditions. In addition, this technique has not until now been capable of adaptation to the examination of real structures cov' ered by a film or skin of photoelastic material, with reflection of the beam of polarized light on this structure. In this case it is impossible to obtain the maximumshear lines or isostatic lines by placing the grid in an analogous way against the surfaceof the real structure examined. A first obstacle is that the light-beam obliged to pass through the grid twice, is attenuated to too great an extent. Moreover, the image of the grid formed relative to the reflecting surface of the observed structure creates extra watered-silk effects which confuse the images. Finally, it is frequently impossible in practice to apply a grid toa non-plane surface.

The presentinvention proposesa process and device which obviates or mitigates the above drawbacks and enables direct visualization of the isostatics or maximum-shear lines more efficiently than in previous techniques, even in the observation of real structures. The present invention can likewise be applied with advantage in the variant in which bytransmission, a model made of transparent photoelastic material is observed.

According to the present invention there is provided I a process of visualizing the maximum-shear'lines or isostatics by observation under polarized light of the accidental double refractions appearing in a photoelastic material, comprising obtaining, successive watered-silk images by observing simultaneously the material and a grid with lines parallel; to the plane of polarization. of the light superimposed for maximum-shear lines, orat 45 to the latter, for the isostatics for different successive orientations of the plane of polarization, one at least of the material. and the, grid being observed in the form of an optical image formed inv the plane of the other.

In a preferred method of-performing the invention the said grid is made up by forming, in the same plane as the material observed, areal optical; image of a grid distant from the material.

In an alternative method applicable tothe observa tion of real structures, the said materialismade up of a photoelastic film observed by reflection of the light on a reflective layer subjacent thereto, and: the real optical image of the grid isformed inthe plane of the refiective layer.

In a modification, the real grid may also be arranged against a screen. on which the image of the material is formed. i

In a preferred. embodiment of thepresent invention, which facilitates exploitation. of the results by enabling I immediate visual observationcontinuous rotation of the polarizer/analyzer/grid andif necessaryth'e screen,

is carried out along with. periodical observation of they image obtained on the screen, superimposed on the grid. For example, a stroboscopic light is used as a light-source. The speed of rotation: and the intervals, or the frequency of the'flashesmay thenbe so selected that thespeed of the successionof the images observed is sufficiently rapidrelative to the retinal inertia,-so that an observer simultaneously seesthe successive images frequency of the flashes being a whole multiple of this number of the order of 40.

By the present inventionthe networks of isostatics or of maximum-shear lines (depending on the orientation of the grid relative to the plane of polarization) result ing from the watered-silk effects between the successive images super-imposed on the grid, are visible directly on a translucent screen without the necessity for photography. In addition, the material may be progressively moved the whole surface of the piece thus being examined in a very short time.

According to another aspect of the present invention there is provided a device for use in visualization of the maximum-shear lines or isostatics by observation under polarized light of the accidental double refractions appearing in a photoelastic material, comprising means defining an optical path between a light source, a photoelastic material to be observed and a screen for observing or recording the images of the said material, a polarizer and an analyzer respectively arranged in the optical path between the source and the material, and the material and the screen, a grid distant from the material and superimposed on the said screen, or associated with means for forming therefrom an optical image in the plane of the material observed, or of its image on the screen, and means of synchronized rotation of the polarizer, the analyzer and the grid relative to the optical path axis.

It is of advantage to arrange the grid against a screen on which there is formed the image of the material, and to arrange the analyzer against the other face of the screen. The grid, the analyzer and even the screen are preferably mounted on the same rotary support on the optical axis, rotated in synchronism with the polarizer.

FIG. 3 shows diagrammatically a network of maximum-shear lines obtained by the visualization process according to the present invention;

FIG. 4 also shows a network of isostatics;

FIGS. 5a and Sb show the application of the processaccording to the present invention to the finishing of a profiled mechanical part; and

FIG. 6 shows diagrammatically a polariscope device according to another embodiment of the present invention.

A polariscope device shown in FIG. 1 is used for the observation of a structure 1, on which there has been placed a reflective layer 2, which is coated with a photoelastic varnish forming a transparent film 3.'The device comprises on thesame side relative to the structure 1, a light-source'4 and a camera photographic plate 5 receiving the image of the source, reflected at a slight angle of incidence on the structure 1, or more precisely by the reflective layer. Along the optical path defined between the light source 4, the structure 1 and the photographic plate 5 via lenses L1, L2, L3 and L4 there are arranged two polarizers whose planes of polarization are crossed, and which constitute respectively the polarizer proper 6 between the source and the structure 1 observed, and the analyzer 7 between the structure 1 and the photographic plate 5.

The light source 4 is at the focal point oflens L4, illuminating the structure by a parallel beam of light. Lenses L2 and L3 focus the light on the polarizer6, and lens L1 focuses the light reflected on analyzer 7, in front of the objective lens 8 of the plate 5.

A grid 10 is arranged in the light-path perpendicularly to the optical axis, before the convergent lens L3. This grid is mounted in a support whose position on the optical axis may be adjusted so that it is at a convergent point of the reflective layer 2, relative to the optical system of lenses L2 and L3.

Otherwise the grid is oriented so that its lines are parallel and perpendicular to the plane of polarization of the light after the polarizer or at 45 to this plane, depending on whether it is required to obtain the maximum-shear lines or the isostatic lines of the structure It is possible to turn the plane of polarization of the light relative to the structure 1, which is presumed to remain fixed by causing synchronized rotation of the polarizer 6, the analyzer 7 and the grid 10, around the optical axis.

The device in FIG. 1a differs from that in FIG. 1 only in the fact that the two beams from polarizer 6 and an alyzer 7' are on opposite sides of the examined model 1' which is cut from a sheet of transparent photoelastic resin, and has neither a reflecting layer 2 nor a film 3 of photoelastic varnish. The function of this device in FIG. la is otherwise identical with that in FIG. 1.

In the variant in FIG. 2, a mirror 11 allows the optical path to be deflected a first time before the polarizer, in order to render the assembly more compact.

In using the device described for visualization of the maximum-shear lines or of the isostatics, the orientation of the grid is adjusted so that its lines are parallel and perpendicular to the crossed planes of polarization of the polarizer and the analyzer depending on the case. Then the grids position on the optical axis is adjusted so as to form a real image of the grid on the reflective layer 2 of the structure 1, causing this real image to coincide with its virtual image relative to the reflective layer. Finally, the camera is focused on this reflective layer.

Several successive photographic exposures are then taken on the same plate, with equal exposure times, each time turning the polarizer/grid/analyzer assembly through the same angle, equal to ln, where n is the number of images photographed, n being advantageously between 3 and I4 and preferably of the order of 9 to l2. Each time there is produced an image of the grid modulated by the network of isoclinic lines for the particular plane of polarization. The image applied to the structure is weak enough for the isochromes other than isochromes of the order of zero to be eliminated. The superimposition of the successive watered-silk effects obtained by the combination of these images directly supply a visualization of the maximum stress lines, as shown in FIG. 3.

By proceeding in the same way but orienting the lines of the grid at 45 to the planes of polarization of the polarizer and of the analyzer, isostatics are obtained as shown in FIG. 4.

Tracings such as those schematically shown in FIGS. 3 and 4 are obtained by observing a piece of beam under load, particularly under the following conditions:

the isostatic tracing. This rectification is shown by FIGS. 50 (before rectification) and 5b (after rectification) in finishing the profile of a pulley mantle.

The rotation of the polarizer of the grid and of the analyzer and their positioning for each photograph taken, may be effected manually or automatically. The rotation may also be continuous, the light-source being made intermittent, at regular intervals, for example, with an electronic flash-gun when passing the desired inclinations, or if necessary, by causing the intensity to vary progressively according to a law of alternative varration.

This possibility will be illustrated in the following with reference to another embodiment of a device according to the present invention described with reference to FIG. 6.

FIG. 6 shows schematically a photoelastic device which comprises, like the embodiments described previously, a polarizer 21 and an analyzer 22, arranged in the optical path of a beam of light produced by a lightsource 23. As the device is here used for observing models made of transparent photoelastic material, the model 24 is arranged between the analyzer and the pola'rizer, the beam of polarized light passing through the model.

An optical system illustrated by a lens-25 enables the image of the model to be formed in the plane of a screen 26 which is translucent'and against which there is arranged a grid 27. This grid may be a simple positive or negative grid, with networks of orthogonal parallel lines. However, in this case it is doubled by another analogous grid whose lines form a variable angle with those of the first. Thewatered-silk effect between the two superimposed grids constitutes a variable-stop grid as a function of the angle formed by the lines of the two grids, and by relative rotation of the latter, the stop may be adjusted at will, which in practice allows the contrast to be improved.

The analyzer 22, screen 26 and the grid 27 (or the grids) are mounted integrally on the same support 28, constituted by a rotary shaft arranged on the optical axis. A synchronizer device 29 when functioning enables synchronization oftheir rotation with that of the polarizer 21 which is likewise rotary on the optical axis.

The screen 26 may be made up of a single sheet of tracing paper. The watered-silk image, obtained by superimposition with grid 27, may be registered on the plate of a camera (not shown). As has already been described above for other embodiments, the network of isostatics or maximumshear lines are then obtained by recording successively on the same plate the watered,- silk images corresponding todifferent-angular positions of the polarizer/analyzer/gridassembly relative to the model. 7 7

However, in the case now described, these images are directly and simultaneously observableon the screen 26. The light-source is then a flashinglight or a strobo scope and the analyzer/polarizer/g'rid-assembly likewise integral with the screen in the special embodiment under consideration, is continuouslyrotated at a speed sufficiently high for it to be possible to illuminate different angular positions relative to the model during the time when the images persist on theobservers retina. The frequency of theflashes is a whole-multiple of the rpm. (revolutionsper'minute) .of this assembly.

By way of example, a rotary speed'of 410 rpm. may be used with a flash frequency of 16,400 flashes per minute.

This method of operation enables the networks of isostatics or maximum-shear lines. to be seen directly such as those shown in FIGS. 3 and 4.

As a variant, the arrangement andfassembly of the grid already described and the same method of operation with direct visualization are equally applicable in the case of observation of a real structure by reflection.

The screen may likewise be stroboscopically illuminated, instead of the photoelastic material.

Naturally, the invention is in no way limited to the ex a screen for receivingthe-images from said material,

a polarizer and an analyzer respectively arranged in the optical path between the source and the material, and the material and the screen, a grid distant from the materialand superimposed on said screen, and 7 means for synchronized rotationof the polarizer, the analyzer andthe grid'relative to the optical path axis to produce a succession-ofsaid imagesat'different angular orientations thereof.

2. A device as claimed in claim 1, whereinthe grid is arranged between thesource and the polarizer.

3. A device as claimed in claim 2, wherein the grid is mounted on a supportwhich is linearly adjustable along the optical path axis.

4. A device asclaimed in claim 1, wherein thepolarizer and the grid are mounted on a common support, which is rotatable about the optical path axis.

5. A device as claimed injclaim 4, wherein the screen is also integral with thesaid support. V

6. A device as claimed in claim 1 whereinsaidlight source is stroboscopic. i

7. A device as claimed in claim 1, wherein said light source is stroboscopic.

8. A device for use in visualization of mechanical stress lines by observation, under a polarized light source, of the double refraction images from a photoelastic material comprising:

means defining an optical path between a light source, a photoelastic material to be observed and a screen for receiving the images from said material,

a polarizer and an analyzer respectively arranged in the optical path between the source and the material, and the material and the screen,

a grid distant from the material and associated with means for forming an optical image, and

means for synchronized rotation of the polarizer, the analyzer and the grid relative to the optical path axis to produce a succession of images at different angular orientations thereof.

9. A device as claimed in claim 8, wherein the grid is arranged between the source and the polarizer.

10. A device as claimed in claim 9, wherein the grid is mounted on a support which is linearly adjustable along the optical path axis.

11. A device as claimed in claim 8, wherein the polarizer and the grid are mounted on a common support, which is rotatable about the optical path axis.

12. A device as claimed in claim 11, wherein the screen is also integral with the said common support.

13. A device for directly visualizing the mechanical stress lines caused by observing, under polarized light, the double refractions appearing in a photoelastic material, said device comprising:

means for producing a moire image by simultaneously observing the material and a grid having lines oriented at a predetermined angle to the plane of light polarization for obtaining mechanical stress lines;

means for rotating said grid and the light polarization plane to obtain moire images from a plurality of angular orientations thereof;

means for receiving said moire images, whereby superimposing all the moire images thereon produces said direct visualization of the mechanical stress. 14. A method of directly visualizing on an image receiving device the mechanical stress lines caused by observing, under polarized light, the double refractions appearing in a photoelastic material, said method comprising:

a. producing a moire image by simultaneously ob- 15.'A method according to claim 14 wherein the step of producing a moire image includes the sub-step of reflecting the polarized light from a reflective layer subjacent said photoelastic material, a real optical image of said grid being formed in the plane of said reflective layer.

16. A method according to claim 14 including forming the successive moire images on an observation screen.

17 The method of claim 14 wherein the step of rotating the grid and the plane of polarization to a new position and the step of producing a subsequent moire image are accomplished at a rate exceeding human retinal analysis.

18. The method according to claim 16 wherein the step of rotating is continuous and the step of producing subsequent moire images is accomplished by strobing the light source. 

1. A device for use in vIsualization of mechanical stress lines by observation, under a polarized light source, of the double refraction images from a photoelastic material comprising: means defining an optical path between a light source, a photoelastic material to be observed and a screen for receiving the images from said material, a polarizer and an analyzer respectively arranged in the optical path between the source and the material, and the material and the screen, a grid distant from the material and superimposed on said screen, and means for synchronized rotation of the polarizer, the analyzer and the grid relative to the optical path axis to produce a succession of said images at different angular orientations thereof.
 2. A device as claimed in claim 1, wherein the grid is arranged between the source and the polarizer.
 3. A device as claimed in claim 2, wherein the grid is mounted on a support which is linearly adjustable along the optical path axis.
 4. A device as claimed in claim 1, wherein the polarizer and the grid are mounted on a common support, which is rotatable about the optical path axis.
 5. A device as claimed in claim 4, wherein the screen is also integral with the said support.
 6. A device as claimed in claim 1 wherein said light source is stroboscopic.
 7. A device as claimed in claim 1, wherein said light source is stroboscopic.
 8. A device for use in visualization of mechanical stress lines by observation, under a polarized light source, of the double refraction images from a photoelastic material comprising: means defining an optical path between a light source, a photoelastic material to be observed and a screen for receiving the images from said material, a polarizer and an analyzer respectively arranged in the optical path between the source and the material, and the material and the screen, a grid distant from the material and associated with means for forming an optical image, and means for synchronized rotation of the polarizer, the analyzer and the grid relative to the optical path axis to produce a succession of images at different angular orientations thereof.
 9. A device as claimed in claim 8, wherein the grid is arranged between the source and the polarizer.
 10. A device as claimed in claim 9, wherein the grid is mounted on a support which is linearly adjustable along the optical path axis.
 11. A device as claimed in claim 8, wherein the polarizer and the grid are mounted on a common support, which is rotatable about the optical path axis.
 12. A device as claimed in claim 11, wherein the screen is also integral with the said common support.
 13. A device for directly visualizing the mechanical stress lines caused by observing, under polarized light, the double refractions appearing in a photoelastic material, said device comprising: means for producing a moire image by simultaneously observing the material and a grid having lines oriented at a predetermined angle to the plane of light polarization for obtaining mechanical stress lines; means for rotating said grid and the light polarization plane to obtain moire images from a plurality of angular orientations thereof; means for receiving said moire images, whereby superimposing all the moire images thereon produces said direct visualization of the mechanical stress.
 14. A method of directly visualizing on an image receiving device the mechanical stress lines caused by observing, under polarized light, the double refractions appearing in a photoelastic material, said method comprising: a. producing a moire image by simultaneously observing the material and a grid having lines oriented at a predetermined angle to the plane of light polarization for obtaining mechanical stress lines, one of said grid and said material being observed as an optical image formed in the plane of the other; b. rotating said grid and the light polarization plane to a new angular position; c. producing a subsequent moire imagE at said new angular position; d. superimposing said subsequent image on all prior moire images; e. repeating steps (b), (c) and (d) at least once.
 15. A method according to claim 14 wherein the step of producing a moire image includes the sub-step of reflecting the polarized light from a reflective layer subjacent said photoelastic material, a real optical image of said grid being formed in the plane of said reflective layer.
 16. A method according to claim 14 including forming the successive moire images on an observation screen.
 17. The method of claim 14 wherein the step of rotating the grid and the plane of polarization to a new position and the step of producing a subsequent moire'' image are accomplished at a rate exceeding human retinal analysis.
 18. The method according to claim 16 wherein the step of rotating is continuous and the step of producing subsequent moire'' images is accomplished by strobing the light source. 